This invention relates to powder coating systems which use corona and tribocharging powder spray guns to apply an electrostatic charge to powder for deposition on a substrate.
There are two basic types of powder spray guns which are commonly used in the electrostatic powder spray coating of articles. The most common type of spray gun is the corona type, which has a high voltage charging electrode which produces a corona to charge the powder. Typically, corona guns are designed to charge the powder negatively. One major disadvantage of corona guns is that they do not coat the interior corners of parts well due to the strong electrostatic field or Faraday caging effect produced by the corona electrode. A second disadvantage to corona guns is that back ionization may occur due to the formation of free ions which results in pinholing or an orange peel surface of the part to be coated. Another disadvantage to these type of guns is that the system components such as the nozzle, and diffuser as well as the powder deliver system components such as the pump, hopper and other parts in contact with the powder delivery system are typically made of materials such as polyethylene or polytetrafluoroethylene (PTFE). While these materials have the advantage of low impact fusion, they have the disadvantage of positively charging the powder, which can impair the negative corona charging process because the final or maximum charge on the powder is diminished. Further, more voltage is often required in order to counteract the positive polarity charging of the system. In addition, this positive polarity tribocharging may cause breakdown of the powder conveying components such as the hose, which connects the pump to the spray gun.
A second type of gun which is also commonly used is a tribocharging gun in which the powder is charged by frictional contact with the interior surfaces of the gun. One advantage to triboelectric guns is that the powder can easily penetrate corners of parts to be coated because the gun does not produce a strong electric field like a corona gun does.
The invention provides novel electrostatic powder coating guns and system components in which powder is pre-charged to the same polarity as a charge applied by the powder spray gun in order to increase and enhance the applied charge and the transfer efficiency. Also novel powder coating methods are described.
In accordance with one aspect of the invention, an apparatus for spraying powder coating material is described. The apparatus has a powder flow path, wherein the powder flow path has a charging surface for triboelectrically charging powder coating material which comes in contact with the charging surface, and the charging surface comprises a negative tribocharging material selected from polyamide resin blends, fiber reinforced polyamides, aminoplastic resins and acetal polymers.
In accordance with another aspect of the invention, an apparatus for spraying powder coating material has a powder flow path, wherein the powder flow path has a charging surface for triboelectrically charging powder coating material which comes in contact with the charging surface, and wherein one or more air passages are formed through the charging surface, the air passages being in a fluid communication with a source of compressed air.
In accordance with another aspect of the invention, an apparatus for spraying powder coating material is described. The apparatus has a powder flow path through which the powder coating material flows, wherein the powder flow path has a first charging surface for triboelectrically charging powder coating material which comes in contact with the first charging surface, the first charging surface comprising a tribocharging material having a first charging polarity, the apparatus further comprising a component through which powder coating material also flows, the component having a second charging surface which also comprises a tribocharging material having the first charging polarity.
In accordance with another aspect of the invention, a system for applying powder coating materials to articles is described. The system includes a powder feed apparatus for supplying powder coating material and an apparatus for spraying powder coating material received from the feed apparatus. The spraying apparatus has an electrode for charging the powder coating material a first charging polarity. The feed apparatus includes a component having a charging surface for triboelectrically charging powder coating material which comes in contact with the charging surface, the charging surface comprising a tribocharging material having the first charging polarity.
In accordance with another aspect of the invention, a system for applying powder coating materials to articles is described. The system includes at least one corona charging spraying apparatus and at least one tribocharging spraying apparatus. The corona charging spraying apparatus has an electrode for charging the powder coating material a first charging polarity. The tribocharging spraying apparatus has a powder flow path, wherein the powder flow path has a charging surface for triboelectrically charging powder coating material which comes in contact with the charging surface, the powder coating material being charged to the first polarity by the charging surface of the tribocharging spraying apparatus.
In accordance with another aspect of the invention, a tribocharging powder spraying apparatus is described. The apparatus includes a body having an internal bore, a wear tube located within the internal bore, and an open passageway provided between the internal bore and the wear tube, with at least one air jet passageway being provided through the wear tube. The air jet passageway provides fluid communication between the open passageway and the interior of the wear tube. The wear tube has a charging surface for triboelectrically charging powder coating material which comes in contact with the charging surface. The open passageway is in fluid communication with a source of compressed air, whereby compressed air flows from the open passageway through the air jet passageway into the interior of the wear tube to affect the flow of powder coating material through the wear tube.
In accordance with another aspect of the invention, a system for applying powder coating materials to articles is described. The system includes a powder feed apparatus for supplying powder coating material and an apparatus for spraying powder coating material received from the feed apparatus. The feed apparatus includes a component having a charging surface for triboelectrically charging powder coating material that comes in contact with the charging surface. The component charging surface is comprised of a negative tribocharging material selected from polyamide resin blends, fiber reinforced polyamides, aminoplastic resins and acetal polymers.
These and other aspects of the invention are herein described in detail with reference to the accompanying Figures.